1. Field of the Invention
The invention relates generally to ultrasonic and radiographic medical imaging, and particularly to three-dimensional, ultrasonic mammography and breast tissue biopsy techniques.
2. Description of the Related Art
Currently, the standard of care for managing breast disease includes breast biopsy to definitively diagnose suspicious lesions. Recently, stereotactically guided breast biopsy techniques have been introduced which allow more accurate guidance of the biopsy instruments, to improve the accuracy of the tissue sampling. It is scientifically, medically, and legally desirable to provide permanent, archivable imagery which documents the tissue sampling, recording precisely the location of each tissue specimen in relation to a suspected lesion and other pathological structures. This task is particularly difficult because the tissue and the lesion are three-dimensional structures, while most imagery used for guidance is two-dimensional.
A traditional method of documenting sterotactic biopsies is to take stereotactic images after a needle has been inserted into a lesion but before the tissue sample is actually taken. If microcalcifications are present, the standard of care specifies that X-ray images must be taken of the tissue samples and that the number of microcalcifications present in these samples must equal the number counted in the original screening mammograms. A free hand biopsy usually includes taking a single X-ray image showing the location of the needle and the lesion. Before and after images are not generally provided.
Interpretation of biopsy-documenting X-ray images is complicated in part because the lesion sampled is a three-dimensional volume, while the image is a projection onto a two-dimensional plane or planes. In addition, the breast may move slightly or be slightly deformed between the image and the biopsy. Another drawback is the need for multiple X-rays of the tissue, which expose the patient to ionizing radiation. Such methods are also inherently inconvenient because the mammograms are not typically immediately available. The patient must wait until the verifying mammograms are produced; and if a post-biopsy mammogram shows that the intended target was missed, additional needle insertions will be required.
Some biopsy guidance methods use ultrasound as an imaging medium to guide a biopsy instrument during insertion. For example, U.S. Pat. No. 5,833,627 to Shmulewitz (1998) describes a method and apparatus for guiding a biopsy needle or the cannula of a biopsy device while inserting it into a tissue mass. His apparatus uses ultrasonography in real time to aid in aligning the biopsy device with the ultrasound image. Similarly, U.S. Pat. No. 5,820,552 to Crosby et al. (1998) describes another apparatus and method which can be used to guide the trajectory of a breast biopsy instrument by employing real time imaging, typically ultrasonography, to enhance accuracy and ease of positioning the instrument.
Ultrasonography is limited in its ability to image certain types of tissues, however, which limits the above described methods. The lower resolution of ultrasonic imaging (compared to x-ray) makes it difficult or impossible to identify fine features, such as hard micro-calcifications in breast tissue, which would be more visible in an x-ray. Imaging of small calcifications is particularly important because such calcifications play an crucial role in the detection of breast cancer. They are frequently the only detectable early sign of breast cancer. Micro-calcifications are typically categorized as either benign, probably benign, or suggestive of malignancy, based on a number of factors including size, shape, and distribution. While some benign calcifications cannot be distinguished from those associated with malignancy, many can be so distinguished by their patterns and distribution. Often these calcifications mark a site which is sufficiently suspicious to merit biopsy.
X-ray mammography is superior in its ability to image microcalcifications, and has been used to guide a biopsy. For example, an x-ray guidance technique is described in U.S. Pat. No. 5,209,232 to Levene and Hadarom (1993). That Patent discloses a system using digital x-ray fluoroscopic imagery, taken from multiple angles, to guide a biopsy needle to its target. This method suffers from at least one obvious drawback: digital x-ray fluoroscopic equipment adequate for that method is quite expensive and bulky. Furthermore, fluoroscopic images are generally considered non-diagnostic (they can direct a biopsy, but lack sufficient resolution for screening or to see microcalcifications).
X-ray mammography also has other shortcomings. This technique provides detailed image information about well differentiated materials (such as bone or other calcified tissue), but it performs poorly at discriminating between soft tissues with subtle differences in density and structure. Some women have mammographically dense breasts, as compared to more fatty breasts. Images from such breasts are generally not clinically useful. The use of x-rays for examination also necessarily results in the exposure of the patient to ionizing radiation, which has well know associated risks. The technique is also limited in that it projects three-dimensional structure onto a two-dimensional plane, and thus does not directly capture the elevation or depth (position in the direction of radiation propagation) of features of interest.
Other biopsy positioning methods and apparatus are known, for example U.S. Pat. No. 5,803,912 to Siczek et al. (1998) and U.S. Pat. No. 5,868,673 to Vesely (1999) (xe2x80x9cA System for Carrying Out Surgery, Biopsy and Ablation of a Tumor or Other Physical Anomalyxe2x80x9d). Vesely""s method requires implantation of an ultrasonic reference transducer, which must be positioned based on at least two mammograms. This method is apparently best suited to tumors of macroscopic size rather than small microcalcifications (or clusters thereof).
Although the aforementioned methods and apparatus aid in obtaining proper biopsy specimens (by guiding the instrument during the biopsy), none of these prior approaches explicitly provides affordable, archivable, easily viewed post-biopsy imagery for easy verification that the biopsy was taken from the precise intended volume.
An image processing system and method visually documents and displays the in-vivo location from which a biopsy specimen was extracted, by processing pre-biopsy and post-biopsy images. A composite image is created which visually emphasizes differences between the pre-biopsy and post-biopsy images. Preferably three-dimensional, digitized images, displayable in various projections, are stored for archival purposes on computer readable media.
To properly relate the pre-biopsy and post-biopsy images, an image processor preferably exploits an optical correlator to register the images accurately, by finding a transformation which produces a pre-determined degree of correlation between the images, then adjusting one image accordingly. The images are then compared, volume element by volume element (xe2x80x9cvoxel by voxelxe2x80x9d), to detect differences between pre-biopsy and post-biopsy images. The composite image is displayed with synthetic colors, synthetic icons, or other visual clues to emphasize probable in-vivo biopsy locations.